This invention relates to a novel method for making an indirectly-heated cathode assembly, and particularly to a method wherein the heater is spaced from the cathode substrate.
A cathode-ray tube, for example a television picture tube, usually employs at least one electron gun with an indirectly-heated cathode assembly. Most color television picture tubes employ three such guns. That type of assembly includes a disc-shaped cathode substrate having an electron-emissive coating on the obverse surface thereof and a grid closely spaced from the coating. A coated-wire resistance heater is closely spaced from the other reverse surface of the substrate. The substrate may be supported on a tubular member, which may also contain the heater. All of the parts are attached to a common support means.
When the heater is operated, it expands due to the heat, and, if it is too close to the cathode substrate, it will press on the substrate and may deform the substrate. Since some of the electrical characteristics of the electron gun, particularly the cutoff voltage, are sensitive to the cathode-to-grid spacing, such deformation is undesirable. This deformation is particularly apparent with stiffer structures such as one using the one-piece bimetal cathode and sleeve described in RCA Technical Note 1159 by J. C. Turnbull, issued July 23, 1976.
U.S. Pat. No. 3,822,392 issued July 2, 1974 to R. J. Bowes et al suggests overcoming this and similar problems by mounting the heater with bimetal pieces which draw the heater away from the cathode substrate when the heater is operated. Such a structure, while it may be effective to overcome the problem, is too expensive and too complex for applications, such as home television, where cost is an important consideration.